Optical recording medium processing device and focal point control method thereof

ABSTRACT

The focus is moved from a first layer (L 0 ) to a second layer (L 1 ) of a double-layered optical disk by stopping a control loop operation and applying acceleration pulses for an interlayer jump to a two-axis electromagnetic actuator. Here, switching timing between acceleration and deceleration for an interlayer jump is generated based on the rate of change of a SUM signal obtained from a light-sensitive element ( 27 ), the SUM signal is distinguished by a specific threshold TH, and an S-shaped error signal is distinguished by thresholds THH and THL with different positive/negative level, thereby performing a pull-in operation of focus control. In addition, at the time of the interlayer jump, a liquid crystal element ( 23 ) for correcting the spherical aberration is optimized in advance for the cover layer thickness of the target layer L 1.

TECHNICAL FIELD

The present invention relates to an optical recording medium handlingdevice and a focus controlling method for the same, the deviceperforming data recording/playback to/from a recording medium such as anoptical disk.

BACKGROUND ART

Hitherto, in optical recording medium handling devices typified by anoptical disk recorder/player, the diameter (φ) of a focal spot isgenerally given according to formula (1):φ=λ/NA   (1)where λ is the wavelength and NA is the numerical aperture of theobjective lens.

Therefore, the shorter the wavelength of the light source and the largerthe numerical aperture of the objective lens, the smaller the diameterof the focal spot on the recording medium and the higher the possibledensity of optical recording.

A focusing objective lens used for an optical data recording/playbackdevice is designed generally so that the residual wavefront aberrationis minimized for a cover layer (transparent protective layer) which isprovided as a protective film on a data-recording layer of a recordingmedium and which has a specific thickness. For example, a focusingobjective lens is designed so as to be the optimum for a cover layerthickness of 1.2 mm in a CD (compact disk) device, and 0.6 mm in a DVD(digital versatile disk) device.

There is provided an optical disk (so-called multi-layer disk) in whicha plurality of data-recording layers are laminated. In this multi-layerdisk, each data-recording layer is covered by a cover layer, the coverlayers having different thicknesses from each other.

In a DVD playback device that plays back a DVD having two data-recordinglayers, the numerical aperture of an objective lens is 0.6, and a redsemiconductor laser with a wavelength of 650 nm is used as a lightsource.

The tolerance of the objective lens to a difference in thickness betweenthe cover layers are given according to formula (2):

$\begin{matrix}{W_{40} = \frac{\Delta\;{t\left( {n^{2} - 1} \right)}{NA}^{4}}{8n^{3}}} & (2)\end{matrix}$where Δt is the difference in thickness between the cover layers, and nis the refractive index of the cover layers (see, for example, S.Kubota, “Aplanatic condition required to reproduce jitter-free signalsin optical disk system”, Appl. Opt. Vol. 26, pp. 3961–3973 (1987)(hereinafter referred to as the Kubota paper)).

For example, when the permissible spherical aberration (W₄₀) is λ/4, thepermissible fluctuation value (Δt) of the cover layer thickness in theabove DVD playback device is ±27 μm. As for the double-layered disk usedin the DVD playback device, the distance between data-recording layersis restricted to about 40 μm so as to fall in the above tolerance.

Recently, there has been disclosed a technique in which a high-capacityoptical disc recording/playback device is achieved by shortening thewavelength of the light source and increasing the numerical aperture ofthe objective lens.

In this technique, a blue-violet semiconductor laser and an objectivelens with a numerical aperture of 0.85 are used, and a recordingcapacity over 23 gigabytes is achieved in a DVD-size optical disk. Onthe other hand, according to formula (2), the accuracy of the coverlayer thickness needs to be within ±4 μm.

However, if the same double-layered disk as in the DVD device is used inan optical disk recording/playback device using a high numericalaperture lens, in order to prevent interlayer interference of datasignals, it is necessary to secure an interlayer distance of about 20μm, which does not fall within the tolerance (±4 μm) for the cover layerthickness.

In order to deal with different cover layer thicknesses, there isdisclosed a technique in which the spherical aberration is corrected byan expander lens (see, for example, Japanese Unexamined PatentApplication Publication No. 2000-131603).

In addition, a more effective technique using a liquid crystal elementis disclosed (see, for example, Japanese Unexamined Patent ApplicationPublication No. 10-020263 and Japanese Unexamined Patent ApplicationPublication No. 2001-331963). The liquid crystal element has, forexample, a concentric electrode pattern, and, according to the voltageapplied to electrodes, it can generate a wavefront substantiallyequivalent to the degree of correction of the spherical aberrationcaused by the thickness error of the cover layers (see, for example, M.Iwasaki, M. Ogasawara, and S. Ohtaki, “A new liquid crystal panel forspherical aberration compensation”, Tech. Digest of Optical Data StorageTopical Meeting, SPIE 4342, pp. 103–105 (2001) (hereinafter referred toas the Iwasaki paper)).

When an objective lens with high numerical aperture is applied torecording and playback of a multi-layer disk medium, it is necessary touse a technique in which the focal point of the focal spot isselectively moved and controlled with respect to the targetdata-recording layer in addition to the above-described correctingtechnique.

There is disclosed a technique in which the above correcting unit isoptimized in advance for the target data-recording layer when thepull-in operation of focus control and the focus movement are performed(see, for example, Japanese Unexamined Patent Application PublicationNo. 2002-100061). In addition to the optimizing technique of the abovecorrecting unit, there is disclosed the application timing ofacceleration pulses necessary for the focus movement (see, for example,Japanese Unexamined Patent Application Publication No. 2002-157750).

In an actual multi-layer optical recording medium, there is apossibility that a manufacturing error concerning the cover layerthickness occurs. Additionally, in the above-described correcting unititself, there is a deviation of the degree of correction made to theapplied voltage. Therefore, after completing the focus control pull-inoperation, there needs to be a fine-tuning so that the degree ofspherical aberration correction is optimal.

In particular, there are disclosed: a technique in which a playbacksignal from a data-recording medium is tuned to the optimal signal byusing the jitter value represented as a fluctuation of the data edgeswith respect to a playback clock (normally, a PLL Clock: Phase-LockedLoop Clock), the signal amplitude, or the error rate (see, for example,K. Osato, I. Ichimura, F. Maeda, K. Yamamoto, and Y. Kasami, “Progressin optical disk recording with over 20 GB of capacity”, Tech. Digest ofOptical Data Storage Topical Meeting, Whistler, pp. 15–17 (2000)(hereinafter referred to as the Osato paper)); and a technique in whichthere is provided an automatic correcting mechanism based on a sphericalaberration error signal generated from the return light intensity from adata-recording medium (see, for example, T. Shimano, M. Umeda, and T.Ariyoshi, “Spherical aberration detection in the optical pickups forhigh-density digital versatile discs”, Jpn. J. Appl. Phys. 40, pp.2292–2295 (2001) (hereinafter referred to as the Shimano paper)).

On the other hand, the wavefront aberration W₂₀ caused by a focus error(Δz) is proportional to the square of the numerical aperture of a lens,and described according to formula (3):

$\begin{matrix}{W_{20} = {\frac{1}{2}\Delta\; z\;{NA}^{2}}} & (3)\end{matrix}$

However, in a multi-layer optical disk device using the above-describedhigh numerical aperture objective lens, since the focal depth of thefocusing objective lens is generally shallow, a focus control errorsignal is discontinuous between data-recording layers. Therefore, thereexists a problem in that it is difficult to determine the switchingtiming between acceleration and deceleration.

Therefore, in comparison with, for example, a known double-layered DVDdevice, interlayer movement of a light spot cannot be achieved easily.

It is an object of the present invention to provide an optical recordingmedium handling device and a focus controlling method for the same thatcan perform interlayer movement from one data-recording layer to anotherdata-recording layer appropriately and easily with respect to arecording medium having a plurality of data-recording layers.

DISCLOSURE OF INVENTION

In order to achieve this object, the present invention includes anobjective lens unit focusing a spot of light onto an opticaldata-recording medium having a plurality of data-recording layers; adriving unit moving the objective lens unit in the direction of theoptical axis to move the focal point of the spot of light between theplurality of data-recording layers; a spherical aberration correctingunit correcting the spherical aberration occurring in transparent coverlayers provided for each data-recording layer and having differentthicknesses; a reflected-light detecting unit detecting the spot oflight reflected by the optical data-recording medium; and a control unitswitching between acceleration and deceleration of the driving unitbased on the rate of change of a reflected-light intensity signaldetected by the reflected-light detecting unit, when an interlayermovement is performed, the interlayer movement being the movement of thefocal point of the spot of light from one data-recording layer toanother data-recording layer.

Furthermore, the present invention provides a focus controlling methodfor an optical recording medium handling device, the device comprisingan objective lens unit focusing a spot of light onto an opticaldata-recording medium having a plurality of data-recording layers, adriving unit moving the objective lens unit in the direction of theoptical axis to move the focal point of the spot of light between theplurality of data-recording layers, a spherical aberration correctingunit correcting the spherical aberration occurring in transparent coverlayers provided for each data-recording layer and having differentthicknesses, and a reflected-light detecting unit detecting the spot oflight reflected by the optical data-recording medium; the methodcomprising the step of switching between acceleration and decelerationof the driving unit based on the rate of change of a reflected-lightintensity signal detected by the reflected-light detecting unit, when aninterlayer movement is performed, the interlayer movement being themovement of the focal point of the spot of light from one data-recordinglayer to another data-recording layer.

In the optical recording medium handling device and the focuscontrolling method for the same according to the present invention,switching between acceleration and deceleration is performed based onthe rate of change of a reflected-light intensity signal from theoptical recording medium when an interlayer movement is performedbetween a plurality of data-recording layers. Therefore, even if thefocus control error signal is discontinuous between data-recordinglayers, acceleration and deceleration of the driving unit can becontrolled appropriately and the interlayer movement can be achievedappropriately and easily.

It is effective especially in the case where playback and recording ofan optical data-recording medium are performed with an objective lenswith high numerical aperture. In addition, in combination with presetvalue control of the cover layer thickness in the spherical aberrationcorrecting unit, it is possible to achieve a reliable focus switchingoperation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing an example configuration of anobjective lens unit of an optical disk playback device in an embodimentof the present invention.

FIG. 2 is a sectional view showing an example configuration of anoptical pickup of an optical disk playback device in an embodiment ofthe present invention.

FIG. 3 is an explanatory view showing an example of an electrode patternof a liquid crystal element used in an optical disk playback device inan embodiment of the present invention.

FIG. 4 is an explanatory view showing an example of a wavefront producedby the liquid crystal element shown in FIG. 3.

FIG. 5 is an explanatory view showing an example configuration of alight-sensitive element divided into areas and used in an optical diskplayback device in an embodiment of the present invention.

FIG. 6 is a block diagram showing the entire configuration of an exampleof an optical disk playback device in an embodiment of the presentinvention.

FIG. 7 is an explanatory view showing an example of an operatingwaveform of an optical disk playback device in an embodiment of thepresent invention.

FIG. 8 is an explanatory view showing the light intensity distributionon the light-sensitive element according to the astigmatism method usedin an optical disk playback device in an embodiment of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

The preferred embodiments of the optical recording medium handlingdevice and the focus controlling method for the same according to thepresent invention will now be described with reference to the drawings.

In this embodiment, when an interlayer movement (interlayer jump) fromone data-recording layer to another data-recording layer of amulti-layer disk is performed, switching between acceleration anddeceleration of a driving unit is performed based on the rate of changeof a reflected-light intensity signal generated by a light intensitydetecting element, thereby achieving an appropriate operation.

Here, as an example, there will be described a technique for performingfocus control and an interlayer jump of a light spot with respect to adouble-layered disk in an optical disk playback device using a two-groupobjective lens with a numerical aperture of 0.85 and a blue-violetsemiconductor laser with a wavelength of 405 nm.

FIG. 1 is a sectional view showing an example configuration of anobjective lens unit of an optical disk playback device in thisembodiment.

The two-group objective lens consists of a first lens 12 and a secondlens 14.

The second lens 14 is mounted on a two-axis electromagnetic actuator 15having a structure capable of moving along the optical axis and in thedirection perpendicular to the signal track. The first lens 12 is heldby a lens holder 13 so as to be located on the same optical axis as thesecond lens 14. These two lenses function as a two-group objective lenswith a numerical aperture of 0.85.

A beam from a semiconductor laser light source (not shown in FIG. 1)passes through these two lenses 12 and 14, thereby being focused on anoptical disk medium 11. Since high numerical aperture is achieved, theoperating distance of the objective lens is small in comparison with ahitherto known optical pickup. In this embodiment, the value is about140 μm.

The increase in numerical aperture of the objective lens generallyreduces the disk tilt tolerance in an optical disk device. When thetilting angle of the disk with respect to the optical axis is θ, thegenerated comatic aberration (W₃₁) is given by formula (4) according tothe Kubota paper:

$\begin{matrix}{W_{31} = \frac{{t\left( {n^{2} - 1} \right)}n^{2}\sin\;{{\theta cos\theta} \cdot {NA}^{3}}}{2\left( {n^{2} - {\sin^{2}\theta}} \right)^{5/2}}} & (4)\end{matrix}$and it is generally proportional to the cube of the NA and the thicknesst of the cover layer of the disk medium 11.

Therefore, when the value of permissible spherical aberration (W₃₁) isλ/4, in order to secure the same disk tilt tolerance in an optical diskdevice with a numerical aperture increased to 0.85 as in a DVD playbackdevice, it is necessary to reduce the cover layer thickness to about 0.1mm. FIG. 1 schematically shows a case where a cover layer 11 a (0.1 tCover) with a thickness of 0.1 mm is provided.

FIG. 2 is a sectional view showing an example configuration of anoptical pickup 10 in this embodiment.

The output light from the semiconductor laser light source 16 isconverted to a parallel beam by a collimator lens 17. The parallel beampasses through a diffraction grating 19, which is for generating sidespots used for calculating a tracking control error signal, and then itis focused on the recording medium by the two-group objective lensformed of lenses 12 and 14. Part of the output light is reflected by apolarization beam splitter 20, and then it is led to a light-sensitiveelement 22 by a focusing lens 21, the light-sensitive element 22 beingused for detecting emission output. This part of the light is used forthe purpose of controlling the laser output at a steady value.

The quantity of light entering the light-sensitive element 22 can becontrolled by rotating a ½-wave plate 18. The actual laser output iscontrolled at a desired emission output value by an automatic powercontrol (APC) circuit (not shown).

Between the polarization beam splitter 20 and the two-group lens formedof lenses 12 and 14, a liquid crystal element 23 for correctingspherical aberration is provided.

FIG. 3 is an explanatory view showing an example of an electrode patternof the liquid crystal element 23 used in this embodiment.

As shown, the liquid crystal element 23 has concentric electrodes 23 a,23 b, and 23 c as disclosed in the above-described Iwasaki paper. Theliquid crystal element 23 can generate a wavefront according to thevoltages applied to the electrodes 23 a, 23 b, and 23 c. The wavefrontis substantially equivalent to the degree of correction of the sphericalaberration caused by a thickness error of the cover layer of the opticaldisk medium 11.

FIG. 4 is an explanatory view showing an example of a wavefrontgenerated by the liquid crystal element 23 shown in FIG. 3. Thehorizontal axis shows radius and the vertical axis shows phasedistribution.

In front of the two-group lens formed of lenses 12 and 14, a ¼-waveplate 24 is disposed. This is for converting a linear polarization ofthe semiconductor laser light source 16 to circularly polarized light.

On the other hand, the reflected light from the optical disk medium 11is reflected by the polarization beam splitter 20, and then it is led tothe detecting optical path.

In this embodiment, an astigmatism method is used as a focus controlerror signal, and a differential push-pull method is used as a trackingcontrol error signal. The converging light passing through a focusinglens 25 and a complex lens 26 enters a light-sensitive element 27, andthen it is converted photoelectrically. The light-sensitive element 27is for detecting a signal functioning as both a servo error signal andan RF signal.

As shown in FIG. 5, the light-sensitive element 27 is a multi-elementdevice having a first light-detecting element 27 a, a secondlight-detecting element 27 b, and a third light-detecting element 27 cdetecting the spot of light. The first light-detecting element 27 a islocated in the center and is divided into four areas. The secondlight-detecting element 27 b and the third light-detecting element 27 care located on either side of the first light-detecting element 27 a andeach divided into two areas. Therefore, the light-sensitive element 27has eight areas in total. Based on the outputs A to H from these areas,the focus control error signal (FE) and the tracking control errorsignal (TE) are calculated according to formulas (5) and (6),respectively:FE=(A+C)−(B+D)  (5)TE=(A+D)−(B+C)−k{(E−F)+(G−H)}  (6)

The RF signal and the SUM signal are given as the sum of the outputs Ato D from the four areas of the first light-detecting element 27 aaccording to formula (7):RF(SUM)=A+B+C+D   (7)The full band component of the signal output is used as the RF signal,and the low band component is used as the SUM signal.

FIG. 6 is a block diagram showing an example configuration of an opticaldata recording/playback device in this embodiment.

First, the playback signal read out from the optical disk medium 11 bythe optical pickup 10 is input into a head amplifier 31. The headamplifier 31 amplifies playback signals from the optical pickup 10(outputs from the divided detecting element) up to a predetermined levelnecessary for processing in a later stage.

The waveform of the playback RF signal amplified here is equalized by anequalizer amplifier (RF EQ) 32, and then it is supplied to a signalprocessing system (not shown).

A DSP (Digital Signal Processor) 39 controls the operation of the entireoptical disk device, i.e., a drive circuit 45 of a spindle motor 46,focusing of the optical system, and tracking.

A focus error signal calculating unit (Focusing Matrix) 33 performscalculation with respect to the input signal according to formula (5). Atracking error signal calculating unit (Tracking Matrix) 34 and a SUMsignal calculating unit (SUM Matrix) 35 perform calculation according toformula (6) and formula (7), respectively.

The calculation outputs are converted into digital signals by A/Dconverters 36, 37, and 38. The DSP 39 performs gain control and phasecompensation concerning the focus control and the tracking control. Theoutputs from the DSP 39 are converted into analog signals by D/Aconverters 40 and 41, amplified by amplifiers 42 and 43 up to thenecessary signal amplitude, and used for driving a two-axiselectromagnetic actuator 15 mounted on the optical pickup 10 in order tocontrol lens position.

The LCD controller 44 controls the liquid crystal element 23 based onthe control from the DSP 39, thereby correcting the sphericalaberration.

Next, there will be described a controlling method in the case where aninterlayer movement of a focal spot is performed with respect to adouble-layered optical disk medium by using the optical pickup 10 ofthis embodiment. The double-layered optical disk medium has a coverlayer with a thickness of 100 μm for a first data-recording layer (Layer0; hereinafter referred to as L0), and another cover layer with athickness of 75 μm for a second data-recording layer (Layer 1;hereinafter referred to as L1).

Here, the current position of the focal spot (that is to say, the focalpoint) is in the L0 layer, and the technique described in JapaneseUnexamined Patent Application Publication No. 2002-100061 is used. Thatis to say, when the focus moves, the cover layer thickness used in thespherical aberration correcting unit is optimized in advance for thetarget layer L1.

FIG. 7 is an explanatory view showing signal waveforms in theoperational examples described below. FIG. 7 shows the change of eachsignal waveform observed when the focus is moved from the L0 layer tothe L1 layer of the double-layered optical disk by stopping the controlloop operation and applying acceleration pulses for interlayer jumpingto the two-axis electromagnetic actuator.

First, FIG. 7( a) shows the SUM signal, and FIG. 7( b) shows the focuscontrol error signal detected at the same time. Here, for example, asshown in FIG. 8, the astigmatism focus error signal shows a circularstrength distribution when light is focused on the light-sensitiveelement 27 (the first light detecting element 27 a) by the complex lens26, and other than that, it shows an elliptical strength distribution(shown by ellipse α (Defocus (+)) and ellipse β (Defocus (−))).Therefore, the calculation result according to formula (4) generates anoutput that becomes zero level when light is focused (so-called S-shapederror signal).

The strength of the S-shaped error signal observed when the focal spotperforms an interlayer movement changes in the sequence: (1) negative,(2) zero, (3) positive, and (4) zero, and the S-shaped error signal isnot continuous. In addition, when the preset value of the sphericalaberration correcting element is considerably different from the optimalvalue for the target recording layer, the focal spot on the recordingmedium is extremely deteriorated by the generated spherical aberration,and there is a possibility that an S-shaped error signal with theoriginal signal amplitude cannot be obtained.

In this embodiment, the switching timing between acceleration anddeceleration for an interlayer jump is generated based on the rate ofchange of the SUM signal, the SUM signal is distinguished by a specificthreshold (TH shown in FIG. 7( a)), and the S-shaped error signal isdistinguished by thresholds (THH and THL shown in FIG. 7( b)) withdifferent positive/negative level, thereby performing a pull-inoperation of focus control.

FIG. 7( c) shows the change in strength (for example, differentialcoefficient) of the SUM signal detected by the DSP 39. On the diagram,the Low level corresponds to the negative differential coefficient, andthe High level corresponds to the positive differential coefficient.

That is to say, since the SUM signal is the minimum when the focal spotpasses through the intermediate position between the L0 layer and the L1layer, it is possible to switch between acceleration pulse applicationand deceleration pulse application by using the changing point of thedifferential coefficient (FIG. 7( g)).

FIG. 7( d) shows a logic signal showing comparison between the SUMsignal and the threshold. FIG. 7( e) and FIG. 7( f) each show a logicsignal showing comparison between the focus control error signal and thethreshold.

Therefore, after switching from an acceleration pulse to a decelerationpulse, when the S-shaped signal exceeds the threshold (THH) and becomesbelow the threshold (THH) again, the application of the decelerationpulse is stopped. When the SUM signal level is greater than or equal toa certain threshold (TH), the focus control loop is operated again.

In this way, an interlayer movement of a focal spot in a double-layeredoptical disk medium is easily achieved in an optical disk device using ahigh numerical aperture objective lens.

Although an embodiment of the present invention is described above, thepresent invention is not limited to the above embodiment.

Although there is described the case where the focal spot moves from theL0 layer to the L1 layer in the above embodiment, it goes without sayingthat an interlayer jump from the L1 layer to the L0 layer can beachieved by using the same technique. In this case, although thepolarity of the observed focus control error signal is generallyreversed, it is only necessary to change the threshold level in theabove embodiment from THH to THL.

In the above embodiment, a technique in which the spherical aberrationcorrecting unit is optimized in advance for the cover layer thickness ofthe target layer L1 is used. Therefore, since a focus error signal withregular strength is obtained at the destination of the focal spot, theoperation timing of the control loop can be easily determined.

By a technique in which the preset value of the spherical aberrationcorrecting unit is preset to an intermediate value between the coverlayer thickness of the L0 layer and that of the L1 layer, there is alsoobtained a focus control error signal with a strength adequate for thepull-in operation determination. Therefore, such a control may beadopted. Although a liquid crystal element is used as an example of aspherical aberration correcting unit in the above embodiment, theabove-described technique disclosed in Japanese Unexamined PatentApplication Publication No. 2000-131603 using an expander lens is alsoeffective.

As described above, in an actual multi-layer optical recording medium,there is a possibility that a manufacturing error concerning the coverlayer thickness may occur. Additionally, in the correcting elementitself, there is a deviation of the degree of correction made to theapplied voltage. Therefore, after completing the focus control pull-inoperation, there needs to be a fine-tuning so that the degree ofspherical aberration correction is optimal.

According to the above-described technique disclosed in the Osato paper,a playback signal from a data-recording medium may be tuned so as to beoptimal by using the jitter value represented as a fluctuation of thedata edges with respect to a playback clock, the signal amplitude, orthe error rate. Alternatively, according to the above-describedtechnique disclosed in the Shimano paper, there may be provided anautomatic correcting mechanism based on a spherical aberration errorsignal generated from the return light intensity from a data-recordingmedium.

Although a two-group objective lens having a high numerical aperture isused in the above embodiment, a single lens can also achieve the samefunction.

Although there is described a device handling an optical disk mediumhaving two recording layers in the above embodiment, an interlayermovement of a focal spot can also be performed in a device handling anoptical disk medium having three recording layers or more and a devicehandling an optical data-recording medium other than an optical diskmedium by using the same technique.

In addition, the present invention can be widely applied to aplayback-only device and a recording/playback device for an opticaldata-recording medium.

As described above, in the optical recording medium handling device andthe focus controlling method for the same according to the presentinvention, switching between acceleration and deceleration is performedbased on the rate of change of a reflected-light intensity signal fromthe optical recording medium when an interlayer movement is performedbetween a plurality of data-recording layers. Therefore, even if thefocus control error signal is discontinuous between data-recordinglayers, acceleration and deceleration of the driving unit can becontrolled appropriately and the interlayer movement can be achievedappropriately and easily.

It is effective especially in the case where playback and recording ofan optical data-recording medium are performed with an objective lenswith high numerical aperture. In addition, in combination with presetvalue control of the cover layer thickness in the spherical aberrationcorrecting unit, it is possible to achieve a reliable focus switchingoperation.

1. An optical recording medium handling device comprising: an objectivelens unit focusing a spot of light onto an optical data-recording mediumhaving a plurality of data-recording layers; a driving unit moving theobjective lens unit in the direction of the optical axis to move thefocal point of the spot of light between the plurality of data-recordinglayers; a spherical aberration correcting unit correcting the sphericalaberration occurring in transparent cover layers provided for eachdata-recording layer and having different thicknesses; a reflected-lightdetecting unit detecting the spot of light reflected by the opticaldata-recording medium; and a control unit switching between accelerationand deceleration of the driving unit based on the rate of change of areflected-light intensity signal detected by the reflected-lightdetecting unit, when an interlayer movement is performed, the interlayermovement being the movement of the focal point of the spot of light fromone data-recording layer to another data-recording layer, wherein theobjective lens unit includes a two-group objective lens with highnumerical aperture.